1. Field of the Invention
The present invention relates to a pressure sensitive current breaker for a cell, which is used for ensuring the safety when a pressure inside a cell case is increased.
2. Description of the Related Art
There has been widely employed a rechargeable lithium ion secondary cell in which an organic solvent system electrolyte is filled and sealed inside a cell case as a power supply for portable equipment, such as portable telephones and personal computers.
However, overcharging during recharging such a cell, or recharging with an excessive current against the specified current will produce abnormality in the cell to generate gas inside the cell case. Therefore, the generated gas raises the pressure and temperature inside the case, swells the case to finally create cracks, and effuses the internal electrolyte, which on some occasion has caused negative influence over the equipment incorporating the cell.
And, even if the abnormality does not reach that degree, to continue using the cell in such abnormality as mentioned above will promote to swell the cell, which leads to such a hazard that the cell case ruptures, and the cell with such abnormality created has to be discontinued in use.
A technique that removes such obstacles will be described with reference to FIG. 8, FIG. 9 which illustrate the conventional pressure sensitive current breaker that is used for a circular cell. The pressure sensitive current breaker includes a cell cover 3 attached to a cell case 1 through a gasket 2 by caulking the surrounding thereof so as to seal the inside of the cell case 1, and, under the cell cover 3, an actuator 4 capable of displacing upward.
The cell cover 3 has an air vent 3a formed thereon, and when the actuator 4 ruptures due to a pressure increase inside the cell case 1, the inner high pressure gas is made to run out through the air vent 3a. 
And, the actuator 4 has an annular safety valve 4a formed by the drawing, etc. The annular safety valve 4a has a protrusion 4b projecting downward on the center of the actuator, and a radial groove 4c formed on the lower side thereof to surround the protrusion 4b. The actuator 4 is made thin in the area where this groove 4c is formed, whereby the actuator 4 can be ruptured by the pressure increase inside the cell case 1.
And, an insulating plate 5 is provided underneath the actuator 4. The insulating plate 5 has a hole 5a through which the protrusion 4b of the safety valve 4a is inserted, and additionally a vent hole 5b formed.
A lead fixing member 6 is provided to underlie the insulating plate 5. The lead fixing member 6 is provided with a hole 6a communicating with the hole 5a of the insulating plate 5, and a vent hole 6b communicating with the vent hole 5b of the insulating plate 5.
Through the holes 5a, 6a of the insulating plate 5 and the lead fixing member 6 is inserted the protrusion 4b of the safety valve 4a, and on the front of the protrusion 4b is attached a thin lead 7 made of a metal plate.
This thin lead 7 has a junction portion 7a welded by the spot welding or the like on the front of the protrusion 4b, whereby the actuator 4 is electrically made conductive with the lead 7. And, the other end of the lead 7 is connected to a generating element 8 illustrated on the lower part in the drawing, and a current path is configured across the generating element 8 and the cell cover 3.
When the pressure inside the cell case 1 is raised by an abnormality of the cell, the pressure increased gas is flown into the vent holes 5b, 6b, as in the arrow A shown in FIG. 9, which produces a force to push up the rear side of the safety valve 4a. 
This force displaces the safety valve 4a upward, and exerts a concentrated stress on the junction portion 7a of the lead 7. This concentrated stress breaks the junction portion 7a to cut off the electric connection between the lead 7 and the actuator 4, thus disconnecting the current path of the cell.
This current path disconnection cuts off the flow of currents inside the cell, so that the pressure increase inside the cell case 1 can be restricted. If the cutoff of the current path does not suppress the pressure increase inside the cell and the inner pressure is further increased, the further increased inner pressure of the cell is exerted on the radial groove 4c formed on the safety valve 4a, the actuator 4 formed thin by the groove 4c is ruptured. Therefore the pressure increased gas runs out through the air vent 3a, thus preventing the blowout of the cell.
Portable equipment such as portable telephones using this type of a conventional cell has advanced miniaturization, and along with this, there has been an increased demand for the miniaturization of a cell used in such portable equipment.
However, in the conventional pressure sensitive current breaker for a cell, the size of the actuator 4 has to be made as large as the actuator 4 can operate without a difficulty, even when the pressure increase inside the cell is low. Therefore, it is difficult to miniaturize the conventional pressure sensitive current breaker, and it has been difficult to miniaturize the cell.
Further, when the lead 7 is ruptured by the pressure increase inside the cell, the junction portion 7a is drawn and stretched by the actuator 4 and broken as it is torn, since the lead 7 is formed by a thin plate.
Accordingly, long burrs like beards are produced on the ruptured face of the junction portion 7a after the rupture, and on the ruptured face of a ruptured hole 7b made after the rupture of the junction portion 7a. Because of the burrs, there has been a possibility that the current path between the actuator 4 displaced up and the lead 7 is not completely cut off.
The present invention has been made in view of the foregoing circumstances, and provides a high-performance pressure sensitive current breaker for a cell that allows the miniaturization and reliably cuts off the current path even when a pressure increase inside the cell is low.
In order to accomplish the foregoing object, the pressure sensitive current breaker for a cell according to one aspect of the invention includes a cell case containing a generating element inside thereof, a cell cover that hermetically seals the cell case, a diaphragm portion formed on the cell cover to expand inward inside the cell case, an external electrode having a terminal part connected to the generating element and exposed outside and a lead part that is easy to break, and an insulating retaining member that embeds the external electrode therein and has an opening to expose the lead part. In the pressure sensitive current breaker thus constructed, the lead part is placed overlying the diaphragm portion to face each other, the lead part located in the opening is formed integrally with an insulator to swell out toward the diaphragm portion, a current path between the generating element and the terminal part of the external electrode is made through the lead part, and when a pressure inside the cell case rises over a specific value, the insulator is pressed by a displacement of the diaphragm portion, and thereby the lead part is ruptured and the current path is cut off.
According to another aspect of the invention, the pressure sensitive current breaker is configured such that the terminal part and the lead part are integrally formed in the external electrode.
According to another aspect of the invention, the pressure sensitive current breaker is configured such that the lead part is embedded into the insulator to be integrated.
According to another aspect of the invention, the pressure sensitive current breaker is configured such that the retaining member is provided with a coupling part to couple the insulator, and the insulator and the retaining member are separated by cutting the coupling part.
According to another aspect of the invention, the pressure sensitive current breaker is configured such that the lead part has parts that extend from both ends of the insulator exposed in the opening, and a notching part formed which thins a wall-thickness of either of the exposed parts.
According to another aspect of the invention, the pressure sensitive current breaker is configured such that the width of the lead part at a part where the notching part is formed is made narrower by another cut.
According to another aspect of the invention, the pressure sensitive current breaker is configured such that when an internal stress that displaces the insulator upward is exerted on the lead part on the far side viewing the insulator from the part where the notching part exposed in the opening is formed and the lead part is broken off at the notching part, the parts where the lead part has been ruptured are significantly separated from each other.
According to another aspect of the invention, the pressure sensitive current breaker is configured such that the insulator has a swollen face expanding toward the diaphragm portion formed in a circular arc, and a projection formed on the swollen face near an edge of the insulator, which is located at the far side from the notching part, and when the diaphragm portion is displaced to break the lead part, the projection is pressed by the diaphragm portion.